Planetary Society solar sail cubesat

[christok]

The Planetary Society says its new "LightSail-1" prototype is ready for launch and promises a major announcement on the 9th of July (10th if you go by UTC). http://www.planetary.org/blogs/mat-kaplan/20140627-lightsail-ready-launch.html

It ought to be interesting.

[duncan1297]

That sounds cool.

Also I think you got the 2000th science labs thread.

[christok]

Also I think you got the 2000th science labs thread.

It feels like something is missing. Perhaps a flashing banner with the promise of a prize once I enter my banking details?

[Scotius]

Really...what's the point? Reaching any sort of useable speed will take much more time than with (still relatively slow) ion engines. And to reach such speed in reasonable time, such probe would have to be really small. Which equals only tiny sliver of mass left for science equipment, power source and communication equipment. For manned flights such propulsion is even more unfeasible. Sure, it requires no fuel - which means weight savings and lower cost. But on the other hand, due to its slowness crew will require much more supplies, living space, radiation protection etc. "Only practical method of interstellar voyage"? PUH-leeeze. I would prefer Orion with all its craziness.

[christok]

Solar sails are hands-down faster than nuclear propulsion for long-distance missions. Low thrust doesn't matter much when the competition has to coast along 99.99% of the time anyway.

[R0cketC0der]

Solar sails don't take power and are cheaper than ion engines.

[Nuke]

i think they might have a couple of issues with deploying and retracting. then there is the problem of damage from space debris. especially the former in the event of the latter. frankly we dont have very much experience with sails at this point. not to say they wont work but it will certainly be a learning experience.

[K^2]

You can get what, 10μN of thrust from square meter under optimal conditions? And you wouldn't be able to go much thinner than 0.01mm for foil. That's about 0.35mN/kg. Pound per pound of propulsion system mass, that's worse than a good ion thruster under the best condition. So solar sail only makes sense when propellant mass is several times heavier than your ion thruster, and that's a hell of a long mission. Since ion thrusters are only getting better and cheaper, the deciding factor is the mass to LEO, and ion gives you much lighter thing to drag to orbit than sails.

Now, if we were to manufacture the probe already somewhere in space. Say, in the asteroid belt, then you really just want a cheap, reliable propulsion system, and solar sails might be a good option there.

[Idobox]

Solar sails are very useful if you are not in a hurry, need an absurd amount of delta-V, and resupply is out of the question. It's a niche, but there are applications:

-interstellar probes (or maybe for outer solar system, ie Kuiper belt or even Oort cloud)

-interplanetary shuttles (low-orbit to low-orbit)

-orbit correction (including dangerous asteroids if we find a way to attach the sail)

It is also possible to use the sail for other things, like pulsed nuclear propulsion (use a "parachute" instead of the Orion pusher plate), power generation, casting a shade, instrumentation (like a gigantic radio-telescope or something).

Finally, with lasers either on the ground or in orbit, you can use beam propulsion and vastly increase the acceleration, and thus applications of the thing.

So yeah, not super sexy, Buck Rogers kind of propulsion, but still worth investigating.

[K^2]

Solar sails are very useful if you are not in a hurry, need an absurd amount of delta-V, and resupply is out of the question.

At anything less than 50km/s delta-V, modern ion thrusters are still better. As in, lighter unit will accelerate payload to desired delta-V in less time. And that's at 1AU, where you'll never need delta-V in excess of 10-20km/s.

Kuiper Belt? That starts at 30AU. So you'll have at best 0.1% of thrust using solar sails. To beat ion thuster there, you'd need target speeds of 400km/s. And again, by the time you'd build up a fraction of that, you'd be so far out of the Solar System that the sail would become absolutely useless.

There is no niche for solar sails. They've become obsolete the moment we started experimenting with ion thrusters.

[peadar1987]

Perhaps as an experiment to test solar sails, this would be good, but the other posters are right, using solar sails to get anywhere at our current level of technology doesn't really make a huge amount of sense.

The only niche I can think of is for something that needs to do a lot of manoeuvres without resupply. K^2 mentioned 50km/s of delta-V. I suppose it's conceivable that we plan a ridiculous mission with a whole bunch of manoeuvres and no resupply that could potentially make that a possibilty.

[Z-Man]

A round trip to 20 asteroids, maybe.

No, I don't know what that would be good for, either. Gathering detailed data to select them and prepare future mining missions?

[Idobox]

At anything less than 50km/s delta-V, modern ion thrusters are still better. As in, lighter unit will accelerate payload to desired delta-V in less time. And that's at 1AU, where you'll never need delta-V in excess of 10-20km/s.

Kuiper Belt? That starts at 30AU. So you'll have at best 0.1% of thrust using solar sails. To beat ion thuster there, you'd need target speeds of 400km/s. And again, by the time you'd build up a fraction of that, you'd be so far out of the Solar System that the sail would become absolutely useless.

There is no niche for solar sails. They've become obsolete the moment we started experimenting with ion thrusters.

I see two missions that could use that kind of deltaV:

Sun dive: Earth orbital velocity is around 30km/s, which you need to get rid of to dive in the sun. And as you get closer, your sail gets better

Flyby of some far KBO, like Sedna. It takes about 12km/s to reach solar escape velocity, but changing plane is going to require a very large deltaV too, possibly more than normal escape velocity.

To reach that kind of velocity, you don't simply thrust prograde. As you noted, the thrust will decrease as you get further.

No, you lower your aphelion as much as possible, and when there, you change plane and get your perihelion. In addition to the Oberth effect, you get significantly more thrust from your sail there.

Once again, not something you will use very often, but there are some niche applications

[Mazon Del]

There is one advantage to solar sails that you don't get with just about any other propulsion system. If you have a microwave laser turret on something like the moon, you can beam out the power at the solar sail and give it some stupidly good acceleration values. NASA did a test in a vacuum chamber here on Earth and managed to get their sample to accelerate with about 10gs straight up.

[K^2]

Idobox, a Sun dive can still be done better with an ion drive. Albeit, not by much. So maybe costs of drive would enter at that point. But good point on inclination change. A mission that might require several major inclination changes can still be done with a lighter unit on a solar sail. Not mission to Sedna, though. A bi-elliptic to Sedna can still be done in well under 50km/s with an ion drive. It's hard to imagine a specific mission that fits the bill, and it's only going to get worse as specific thrust of the ion thrusters improves. But I'll give you that there can, in principle, be such a mission in solar system.

[AngelLestat]

i think they might have a couple of issues with deploying and retracting. then there is the problem of damage from space debris. especially the former in the event of the latter. frankly we dont have very much experience with sails at this point. not to say they wont work but it will certainly be a learning experience.

You dont need retract a solar sail. You use inflatable boomb as solar sail structure. The tanks and mechanism to inflate booms and deploy the sail are detached after used as it show in this video:

You can get what, 10μN of thrust from square meter under optimal conditions? And you wouldn't be able to go much thinner than 0.01mm for foil. That's about 0.35mN/kg.

Yeah, 9 micro newton by square meter at 1Au. But the thickness of kapton foil (5g/m2) which is used in Solar sails is 0,0005 mm. For example SunJammer the solar sail that it will be launched at January 2015, has 1200m2 of sail surface; that surface weights 6kg.

What do you do with 6kg of xenon gas in a ion thruster?

Pound per pound of propulsion system mass, that's worse than a good ion thruster under the best condition. So solar sail only makes sense when propellant mass is several times heavier than your ion thruster, and that's a hell of a long mission. Since ion thrusters are only getting better and cheaper, the deciding factor is the mass to LEO, and ion gives you much lighter thing to drag to orbit than sails.

Is not fair to compare, because ion engines has many years of development. Solar Sails just started and they had a lot of ground to improve their efficiency.

For example you can have a quarter-wave CNT sail, this would be several orders of magnitude lighter than kapton. This can skyrocket your weight/thrust ratio.

At anything less than 50km/s delta-V, modern ion thrusters are still better. As in, lighter unit will accelerate payload to desired delta-V in less time. And that's at 1AU, where you'll never need delta-V in excess of 10-20km/s.

If you close to the sun the acceleration that you get from solar sails increase by the square rule.

1360w/m2 at earth; 2700w/m2 Venus; 9700w/m2 Mercury; 0,1Au 136000w/m2; 0,01AU 13600000w/m2=0,1N/m2

Idobox, a Sun dive can still be done better with an ion drive. Albeit, not by much. So maybe costs of drive would enter at that point. But good point on inclination change. A mission that might require several major inclination changes can still be done with a lighter unit on a solar sail. Not mission to Sedna, though. A bi-elliptic to Sedna can still be done in well under 50km/s with an ion drive. It's hard to imagine a specific mission that fits the bill, and it's only going to get worse as specific thrust of the ion thrusters improves. But I'll give you that there can, in principle, be such a mission in solar system.

Eh? can you elaborate that?

WIth a solar sail you brake free in your sun dive, then in your periapsis with the obber effect and the high w/m2 (which ion with normal PV can not take advantage of) you get max acceleration, then you can still use your sail to keep accelerating or maneuver.

About the uses of Solar sails are many. Even in their current develpment stage.

SunJammer will act as an early sun warning system, in addiction it would carry several instruments to do other things (becoz other missions are planned after that), it also carry ashes from the star trek creator and his wife

Solar sails are the best option to debris dispossal and satellite decommission. There is one called Gossamer, this solar will use a test to reduce the orbit of the old satellites (using solar pressure and the same sails as drag), it would be launched this year and another test for the 2015.

These test use small solar sails 5m X 5m packaged in 15cm3, is enoght to decommision a 700kg satellite.

The main goal of this, it would be had 10 or 20 a reusable solar sails as orbit garbage collector lowering the orbit of objects to a certain altitude when after that they would fall alone by drag in the next 10 years (instead 100 years as it can take)

A 200m x 200m solar sail with a 0,1g/m2 (a weight ratio that we can achieve even today with ease) would weight no more than 10kg with instrument payload of 20 kg can reach 33km/s in one month at 1AU. With something like this you can explore the whole solar system in few years.

Edited July 1, 2014 by AngelLestat

[christok]

What do you do with 6kg of xenon gas in a ion thruster?

That's not how you do the comparison. You should compare the total propulsion system to the total propulsion system. The mass of a solar sail plus all related structures (booms, tethers, whatever else you use) should be compared to the propellant, tank, engine and, importantly, solar panels needed to run an ion engine. They need a lot of juice and you need to include it in the mass budget.

[Kinglet]

Fire massive orbital lasers at the sail.

[K^2]

AngelLestat, when you show me a sail that can actually unfold at 6kg/1,200m², then we talk. Right now, the support structures weigh dramatically more. In fact, the total for the SunJammer is 32kg. See the problem? That brings the total specific thrust to 0.34mN/kg. Same as aluminum foil.

So lets see, SunJammer's sail is 32kg generating 10.8mN of thrust at the most. A modern ion thruster can generate the same with a 10kg unit. That leaves me not 6kg, but 22kg for Xenon. What can a 10kg rocket with 22kg of xenon do? About 60km/s. Does SunJammer's mission involve total delta-V over 60km/s? I did not think so. Same probe with ion thruster could have done mission with a cheaper launch.

And yeah, you can make up all sorts of future materials that would boost solar sail's specific thrust. Except that we aren't anywhere close to the limit on ion thrusters either. And ion thrusters have way more room to improve their specific thrust. Solar sails cannot physically catch up to ion thrusters on that.

If you close to the sun the acceleration that you get from solar sails increase by the square rule.

Yeah, except you have to spiral in at 70% efficiency (45° angle), killing most of your velocity at high altitude, simply because of how low the thrust is. Why don't you do the actual math on trajectory something like SunJammer would take on a Sun dive, and then compare it to an ion thruster. I can match that trajectory with ion thruster at a fraction of the total weight. That's a fact.

WIth a solar sail you brake free in your sun dive

It's not free if you had to launch a 32kg sail into LEO to do a job that could have been done with a much lighter ion thruster unit.

[Idobox]

K^2, we all agree that current solar sail technology can't compare with ion drives, but that doesn't mean we should stop working on it just because the materials that would make it useful are not available yet.

A sail made of graphene or boron nitride would be much lighter. Because the forces are so small, you could keep the shape without using physical supports. For example, electrostatic charges on the rim could be enough, or running current in loop, also on the rim. Another option would be to make an inflatable sail, or inflatable support arms, you don't need a lot of pressure to keep the thing straight.

[AngelLestat]

AngelLestat, when you show me a sail that can actually unfold at 6kg/1,200m², then we talk. Right now, the support structures weigh dramatically more. In fact, the total for the SunJammer is 32kg. See the problem? That brings the total specific thrust to 0.34mN/kg. Same as aluminum foil.

That is the total with the payload. I dont know what is the weight of the boombs/structure, but I guess that the payload was close to 20 kg.

Now I can not find that info.

But what can you do with 15 kg for your ion engine plus the same payload?

As christok said, you need to count the ion engine weight, the tanks weight, propellent and solar cells.

So lets see, SunJammer's sail is 32kg generating 10.8mN of thrust at the most. A modern ion thruster can generate the same with a 10kg unit. That leaves me not 6kg, but 22kg for Xenon. What can a 10kg rocket with 22kg of xenon do? About 60km/s. Does SunJammer's mission involve total delta-V over 60km/s? I did not think so. Same probe with ion thruster could have done mission with a cheaper launch.

´

Plus the solar cells... Or where you get your energy for the ion thruster?

As I said, 32 Kg is the total weight of sunjammer, payload(several instruments, ashes, solar cells, etc) + structure-servo controls + sail.

The sail is 8kg (is not 6, I was wrong.), the structure is the same as the sail 8kg extra. Then the payload is 15 kg.

Here explain than you need to take the double for the structure, For example if you have a sail dencity of 5g/m2, then with the support structure it will be 10g/m2.

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/18379/1/99-1857.pdf

About how to calculate the deltaV of a solar sail, we cant, it will be infinite in theory. Of course a small solar sail would had low acceleration, but you solve that just with scale.

Ion thrusters are cheaper for now. And they always will be a good choice depending your mission.

But solar sails are very usefull for many applications.

And yeah, you can make up all sorts of future materials that would boost solar sail's specific thrust. Except that we aren't anywhere close to the limit on ion thrusters either. And ion thrusters have way more room to improve their specific thrust. Solar sails cannot physically catch up to ion thrusters on that.

mmm I am not so sure.

Thanks to solar sails it will be possible to sent a interstellar solar probe maybe in 50 years (if they take the compromise serious as when we go to the moon) to centauri system with 5% to 10% the speed of light just using the sun as energy source, and it can brake using a similar manuver when it arrives.

it will be by far the most cheap way to do it.

Yeah, except you have to spiral in at 70% efficiency (45° angle), killing most of your velocity at high altitude, simply because of how low the thrust is. Why don't you do the actual math on trajectory something like SunJammer would take on a Sun dive, and then compare it to an ion thruster. I can match that trajectory with ion thruster at a fraction of the total weight. That's a fact.

But you can do it all the time. You ever tried solar sails in ksp with the interstellar mod? I remember that the average density of that sail was really bad and even with those numbers was too easy to fall toward sun.

The trajectory calculations for solar sails takes time (even using the 2d formule), I need to see if I can find an addom for the matlab.

It's not free if you had to launch a 32kg sail into LEO to do a job that could have been done with a much lighter ion thruster unit.

Yeah, you still believe that the 32 kg represent only the sail and the structure, but is also the payload. But maybe the word that you are looking for is "could have been done with a cheaper ion thruster".

And I have to disagree, becouse one of the sunjammer mission is stay in a virtual lagrange position to provide extra warning system of solar anomalies. How much time you can burn with a ion thruster to keep that position?

Edited July 2, 2014 by AngelLestat

[christok]

As christok said, you need to count the ion engine weight, the tanks weight, propellent and solar cells.

To be completely fair, I should actually have said additional solar panels. The payload will usually require less power while en route so you can use some of that power--but electric propulsion is still comparatively power-hungry.

In any case, electric propulsion and solar sails aren't a case of either/or. One proposed way to do an interstellar precursor mission* is to use an ion drive to lower your perihelion, deploy a solar sail very near the sun, ditch the sail after a final solar-powered orbit adjustment about 5 AU out and then switch to the ion drive for any further adjustments. Or alternatively keep the sail, ditch the ion drive and point a big laser at the sail. The initial sunward dive would take too long to execute with the sail only (you don't actually want to lower your aphelion).

* Refer to W. Seboldt & B. Dachwald, Solar sails--propellantless propulsion for near- and medium-term deep-space missions. 2008. (In C. Bruno, A.G. Accettura, Progress in astronautics and aeronautics, vol. 223: Advanced propulsion systems and technologies, today to 2020. 2008. American Institute of Aeronautics and Astronautics. ISBN 978-1-56347-929-8. pp. 443-445) for a discussion of how this could be used to study the Pioneer Anomaly. That particular problem has been solved but the proposal remains interesting.

[AngelLestat]

you can do it with only a sail, You would use jupiter to lower at max your periapsis and fall from there, It will take time, but how much time would take an interstellar mission?

But it has one inconvenient, if the sail is too light, even if you place it parallel to the sun light, you get some counter push when you are falling toward sun because light does not come from a infinitesimal point (sun diameter).

The ion option it would not work either in case you wanna a very close encouter, becouse you can not deploy the sail at those light pressures.

If you dock something heavy to the sail it might work, then undock at periapsis.

[christok]

you can do it with only a sail, You would use jupiter to lower at max your periapsis and fall from there, It will take time, but how much time would take an interstellar mission?

I said interstellar precursor mission. That's a mission to the interstellar medium, not a mission to other stars. Every year you add to the mission time increases complexity, cost and component failure probablities. Let's just say that spiraling out to Jupiter doesn't take a year or two on a solar sail.

The ion option it would not work either in case you wanna a very close encouter, becouse you can not deploy the sail at those light pressures.

You make assumptions about the type of sail and method of deployment.

If you dock something heavy to the sail it might work, then undock at periapsis.

That would negate all mass benefits of using the sail.

[AngelLestat]

I said interstellar precursor mission. That's a mission to the interstellar medium, not a mission to other stars. Every year you add to the mission time increases complexity, cost and component failure probablities. Let's just say that spiraling out to Jupiter doesn't take a year or two on a solar sail.

For a precursor mission you dont need to do a so close periapsis, so maybe it would not be trouble.

You make assumptions about the type of sail and method of deployment.

My assumptions are based in all the thing I read about solar sail and my common sense.

I am not saying that is impossible, but if there is a method to deploy a sail under those circustances I never read it and is not easy to imagine either.

If you have some ideas how it could be share it.

with a 0,007 Au periapsys, the peek acceleration that the sail would had is about 50000 Gee, the sail can survive that if it has a total flat area and thin structure with all their instruments imprinted in the sail it self, but if you try to deploy under those pressures, it would break for sure.